This invention in general relates to compact, folding, single-lens-reflex cameras for "print photography" and in particular to viewfinders for such cameras.
Cameras for use with film formats of modest size, diagonals of three to five inches more or less, often employ folded optical paths with minimal camera body structure to achieve compactness. In addition, every effort is made to minimize the size of required components or to consolidate their functions in the fewest possible elements wherever possible without sacrificing performance and incurring most penalties.
Several prior-art, single-lens reflex print cameras accept a print film pack in a film compartment at the unit's base. The sensitive surface of the film extends parallel to the axial light rays entering the objective. To permit an observer to view a scene prior to exposure, a fixed diagonal reflex mirror reflects light passing through the objective onto the upper surface of a movable capping plate that overlies the film compartment parallel to the film surface. The capping plate keeps the film compartment impervious to light, and its upper surface serves as a reflective focusing screen. A viewfinder utilized the light reflected by the focusing screen to form an observable image.
For film exposure, the capping plate that keeps the film container safe from unwanted exposure pivots from the film-covering "down" position to a film-exposing diagonal "up" position at which it covers the fixed reflex mirror. In its "up" position, a movable reflex mirror surface on the lower face of the plate reflects light passing from the objective lens onto the film. The objective lens then focuses light onto the film.
In such cameras, viewfinders form erect, unreverted images while compensating for the various oblique reflection surfaces within the imaging chamber and orient the viewfinder itself relative to the image on the focusing screen of the capping plate. U.S. Pat. Nos. 3,783,764 and 3,836,931, assigned to Polaroid Corporation, describe folding cameras whose viewfinders furnish erect, unreverted images, reduce the accommodation required of a viewer, and minimize field tilt. They accomplish this by having the fixed reflex mirror deflect light from the focusing screen out of the imaging chamber into a viewing chamber with a two element viewfinder or a three element viewfinder. In the viewfinder, an aspheric concave mirror reflects the light through an eye-lens having an aspheric surface. The concave mirror is a portion of a surface of revolution about an axis. The meridional cut of the concave mirror is defined mathematically by the expression given in the patents, being first a standard form of conic expression of high eccentricity followed by an array of power series terms.
A similar mathematical expression defines the rear surface of the eye-lens, that is, the surface nearest the eye of the observer, with first the conic expression, next aconic terms of 4th and 6th orders, and last further power series terms through the 6th order.
However, the three element viewfinder adds undesirable complexity and cost to a camera arrangement where space is at a premium while the two element viewfinder gave unacceptable images which resulted in a visual field that "swan" with transverse and longitudinal eye movement.
Therefore, a primary object of this invention is to improve single lens reflex cameras and their viewfinder systems.
Another object of the invention is to improve viewfinders for single lens reflex cameras.
Yet another object of the invention is to improve compact folding single lens reflex print cameras and their viewfinders.
Yet another object of the invention is to avoid the aforementioned deficiencies.
Other objects of the invention in part will be obvious and in part will appear hereinafter. The invention accordingly comprises the apparatus possessing the construction, combination of elements, and arrangement of parts which are exemplified in the following detailed description.